The conventional known vibration isolating device widely used, for instance, as an engine mount for automobile vehicles comprises a frame body connected to a vehicle chassis, a frame body connected to an engine, an elastic member disposed between these frame bodies and contributing to the formation of a closed chamber, water or other liquid filled in the closed chamber, a partition member secured at its peripheral edge to one of the above two frame bodies and dividing the closed chamber into two liquid chambers, and a passage arranged in the partition member to communicate the two liquid chambers with each other.
When vibration from the engine is transmitted to the vibration isolating device, it can be damped by a flow resistance subjected to the liquid when the liquid flows from the one liquid chamber to the other through the passage in addition to an internal friction of the elastic member.
In this vibration isolating device, however, when the vibration transmitted to the device is, for example, high frequency, small amplitude vibrations of not less than 50 Hz, the communication passage between both the liquid chambers is rendered into a choked state to raise the dynamic spring rate in the device, whereby the ride comfortability on the vehicle is unfavorably deteriorated.
In order to solve the aforementioned problem, there has been proposed a vibration isolating device in Japanese Patent laid open No. 53-5,376 (Japanese Patent Application No. 52-78,477), wherein an oscillation plate is arranged in the boundary between both the liquid chambers. In this case, if high frequency, low amplitude vibrations are transmitted to the vibration isolating device, the oscillation plate is oscillated in the liquid to absorb such vibrations.
In this device, however, the oscillation plate absorbs the high frequency vibrations because of oscillation in the liquid. The state of no oscillation of the plate results from the friction force between the oscillation plate and the liquid, inertia force of liquid to be removed by the oscillation plate during the oscillation, i.e. rapid increase of dynamic spring rate is unfavorably caused at a relatively low frequency of 150-200 Hz in the high frequency zone.